Numerous positive displacement pumps are known in which the substance to be measured out comes into contact with one of the sides of a piston that is movably mounted in a cylindrical pump chamber.
In pumps of that type, the other side of the piston is in contact with the ambient medium and there is a major risk of the substance which is being handled becoming contaminated. To mitigate that drawback, it is necessary to provide a bactericide or sterile water on the side of the piston that is contact with the ambient medium, thereby requiring the pump to be relatively complex in structure, in particular at the piston sealing ring, in order to prevent the bactericide or the sterile water passing to that side of the piston which is in contact with the substance to be measured out.
Positive displacement pumps are also known that include an admission opening at one end of the pumping chamber and a delivery opening at an opposite end thereof, the piston then having a pumping opening passing therethrough which is alternately opened and closed by means of a non-return valve. The substance to be packaged then comes into contact with both faces of the piston so that the danger of contamination exists only relative to the piston rod, where such risks are relatively easily contained. Nevertheless, the presence of a non-return valve that is not accessible without disassembling the pump makes the operations of cleaning and sterilizing the pump complex. In addition, because of the non-return valve, the pump can be operated only with liquids that are free from particles and pieces of matter, since they could jam the non-return valve in an open position.